Efficient Charge Transport Driven by Strong Intermolecular Interactions in Cyclopentadithiophene-Based Donor–Acceptor Type Conjugated Copolymers

Abstract

Understanding charge transport in π-conjugated polymers is critical for the design of high-mobility polymers. However, charge transport in donor–acceptor (D–A) type copolymers and the roles of D and A units are still unclear. Herein, the charge transport properties of three cyclopentadithiophene (CDT)-based D–A type copolymers with CDT as a common donor and benzothiadiazole, fluoro-2,1,3-benzothiadiazole, or pyridyl-2,1,3-thiadiazole (PTz) as an acceptor are investigated. The best charge transport is found for CDT-PTz showing 0.79 ± 0.01 cm2 V−1 s−1 hole mobility in field-effect transistors, with the lowest energetic disorder by temperature-dependent mobility measurements. Using various structural probes and density functional theory simulations, it is found that CDT-PTz has the most rigid and planar backbone structure and the strongest intramolecular dipole moment thanks to the noncovalent N–S interactions and the strong electron-withdrawing PTz acceptor unit. These structural properties lead to strong intermolecular interactions in CDT-PTz, reducing its π–π stacking distance, producing strong aggregation even in hot dilute solutions. The enhanced mobility in CDT-PTz is attributed to this high quality π–π stacking, driven by increased backbone rigidity and strong molecular dipoles. This work suggests that rigidifying the copolymer backbone with enhanced push–pull strength of the D–A units is a key requirement for high-mobility D–A copolymers.